An intake and an exhaust valve in an engine combustion chamber of automobiles and other motor vehicles comprises a valve body, a valve stem extending therefrom, and the farthest end of the valve stem. A valve of this type is usually manufactured, for example, by cutting a steel rod having a diameter of 7 mm into a length of 250 mm. After upset-forging one end of the cut-length bar with electric heating (a process known as electrothermal upsetting), a mushroom-shaped valve body is roughly formed by hot die-forging. The semi-finished blank is finished to the desired final shape by applying stress-relief annealing, machining, grinding and surface treatments to provide wear resistance, such as soft nitriding, as requited.
The face, stem and stem end of engine valves are required to have adequate wear resistance. Because of their service environment, engine valves must have high-temperature strength, corrosion resistance and oxidation resistance. For this reason, conventional engine valves have generally been made of heat-resisting steels.
On the other hand, in recent years, there has been increasing demand for lighter engines to improve fuel consumption without lowering power output. Weight reduction of engine valves moving up and down at high speeds provide great contributions to the improvement of fuel consumption. Therefore, various attempts have been made at the use of titanium alloys having high specific strength. For instance, Ti--6Al--4V alloy, a typical example of the .alpha.+.beta. type titanium alloys, has been extensively used for the manufacture of intake valves of racing cars. However, engine valves made of titanium alloys will not have high enough durability to withstand the abrasion resulting from the friction with the valve seat, guide and other parts if no improving treatment is applied. Though, therefore, conventional engine valves of titanium alloys are manufactured by the same method as those of heat-resisting steel, for example, molybdenum is sprayed onto their stems to impart high wear resistance. This additional process of molybdenum spraying is costly and uneconomical.
Other methods for imparting wear resistance to engine valves of titanium alloys have been also proposed, such as ion nitriding disclosed in Japanese Provisional Patent Publication No. 234210 of 1986, non-electrolytic nickel alloy plating disclosed in Japanese provisional Patent Publication No. 96407 of 1989, ion plating and nitriding disclosed in Japanese Provisional Patent Publication No. 81505 of 1986 and oxide scale formation disclosed in Japanese Provisional Patent Publication No. 256956 of 1987.
Each of these methods has its advantages and disadvantages. In non-electrolytic nickel alloy plating, for example, the oxide film that unavoidably forms on the surface of titanium alloy impairs the adherence of the coating. To avoid this impairment in coating adherence, the oxide film must be removed by such methods as shot blasting and pickling in fluoric acid. Otherwise, the impaired coating adherence must be improved by applying post-plating diffusion heat treatment. However, none of these corrective actions is favorable. Ion-plating is unsuited for mass-production because of its equipment limitations.
Oxidizing and nitriding in suitable environments are known to impart wear resistance at a relatively low cost. However, the heating at high temperature involved in these processes causes thermal deformation (especially the bending of valve stems) of valves made of the .alpha.+.beta. type titanium alloy, thus defying the attainment of the desired configurational and dimensional accuracies. This problem may be solved by repeated strengthening of the stem or preparation of larger semi-finished blanks to allow the removal of deformed portions. However, these remedies are unfavorable and inefficient because titanium alloys are expensive and difficult to machine, as is described in page 74, No. 2, Vol. 35 of "Titanium and Zirconium." The configurational and dimensional changes are due to a very small creep deformation (approximately 2.times.10.sup.-6 %) which a titanium alloy valve undergoes under the influence of a slight strain caused by its own weight (approximately 50 g) when it is subjected to oxidizing or nitriding at a temperature of 700.degree. C. to 900.degree. C.
Japanese Provisional Patent Publication No. 28347 of 1989 discloses a method for improving the creep properties in service environments of engine valves made of the .alpha.+.beta. type titanium alloys. This method necessitates rendering the microstructure of the valve body into one consisting of finely dispersed acicular .alpha. crystals. Such a microstructure is obtained by prohibiting the formation of equiaxed .alpha. crystals by working the stock with a forging ratio of 2.5 or under in the .alpha.+.beta. phase forming temperature zone after air- or water-cooling from the .beta.-phase temperature zone.
Because of the need to limit the degree of working, this method separately fabricates the valve body and stem, then joins them together at a low enough temperature to prevent the destruction of the built-in microstructure, with the soundness of the produced joint subsequently inspected. Obviously, the process involving all these steps cannot be very efficient.
An object of this invention is to provide titanium alloy bars suited for the manufacture of engine valves whose valve body and stem can be integrally fabricated by conventional electrothermal upsetting. Titanium alloy bars according to this invention permit economical mass-production with less machining or grinding allowance than before as they do not cause significant dimensional and configurational changes (especially the bending of valve stems) when they are heated to high temperatures in stress-relief annealing. The economical oxidizing or nitriding process to impart the desired wear resistance can be also applied to the finished blanks made of this invention bars without dimensional and configuration changes.
Another object of this invention is to provide titanium alloy bars having good cold workability required in the manufacture of themselves.